1. Field of the Invention
The present invention relates generally to a wireless power receiver, and more particularly, to a wireless power receiver for wirelessly receiving power based on an electromagnetic resonance method.
2. Description of the Related Art
Mobile terminals, such as cell phones and Personal Digital Assistants (PDAs), are powered by naturally rechargeable batteries, and supply electric energy to the battery using a separate charging device in order to recharge the battery. Typically, the charging device and the battery are electrically connected by a connection of their outer-surface terminals to each other.
However, when using such a contact charging method, the contacting terminals are susceptible to contamination by dirt because they extend outward, thus suffering from inappropriate charging, and they may not be properly charged when exposed to moisture.
To address the problem, wireless charging or contactless charging technologies have recently been developed and applied to various electronic devices.
The wireless charging technology using wireless power transmission and reception enables, for example, a battery of a cell phone to be automatically charged by placing the cell phone on a charging pad without requiring a separate charging connector. This method is currently applied to wireless electric toothbrushes or a wireless electric shaver. From the wireless charging technology, the electronic device may benefit from enhanced waterproof and portable functions since no wired charging device is required. This technology is expected to be further developed in the burgeoning era of electric vehicles.
The wireless charging technology has an electromagnetic induction method using coils, a resonance method using resonance, and a Radio Frequency (RF)/micro wave radiation method that converts electric energy into microwaves for transmission.
Although it has thus far been dominated by the electromagnetic induction method, recent successful experiments of microwave-based wireless transmission from a few tens of meters a manner, both at home and abroad, indicate that all electronic products will be wirelessly recharged at any time and place, in the near future.
A power transmission method based on electromagnetic induction is based on transferring power between primary and secondary coils. Moving a magnet through a coil produces an induced current based on which a magnetic field is produced at the transmission end, and the change in the magnetic field at a receiving end induces a current to generate energy. This phenomenon is referred to as magnetic induction, and the power transmission method based on the magnetic induction provides superior energy transmission efficiency.
With respect to the resonance method, Soljacic from the Massachusetts Institute of Technology (MIT) suggested a system to which electricity is delivered wirelessly even when the system is a few meters away from a charging device, using a resonance-based power transmission principle based on Coupled Mode Theory. The MIT team's wireless charging system is based on the resonance effect, a physical concept that a tuning fork being placed next to a wine glass causes the wine glass to ring with the same frequency. Electromagnetic waves carrying the electric energy are resonated instead of the sound. Resonant electric energy of the electromagnetic waves is directly transferred only when there is a device having the same resonant frequency and the non-used part of the energy is re-absorbed into the magnetic field rather than being dispersed in the air, and thus the resonant electric energy is not deemed harmful to surrounding machines or bodies.
However, a wireless power receiver based on the conventional resonance method includes a rectifier that converts a received Alternating Current (AC) into a Direct Current (DC) and a DC-to-DC converter that adjusts the rectified DC voltage to a voltage preset for an output end. However, since the DC-to-DC converter must use a bulky passive element, it is difficult to implement into the conventional method the wireless power receiver that satisfies both compactness in appearance and high output performance with high efficiency. Particularly, when the wireless power receiver is implemented in a mobile communication device such as a cell phone, such difficulty detrimentally affects the current trend of lighter and smaller mobile devices.
Furthermore, the DC-to-DC converter uses a frequency in a limited range of 2 to 3 MHz since it is an outwardly packaged active device.
In addition, the conventional wireless power receiver suffers from having to include an additional capacitor, such as for impedance matching, and an additional Zener diode for protection of the entire device.